Printing screens, frames therefor and printing screen units

ABSTRACT

A printing screen unit including a printing screen having two pairs of opposite edges and a frame including interface members bonded to opposite edges of the printing screen and coupled together at corners of the frame, wherein the frame holds the printing screen in an untensioned state when no tension is applied thereto by external tensioning mechanisms separate to the frame, and the frame is substantially rigid when in the untensioned state, but allows for relative movement of the respective pairs of interface members by a tension applied by external tensioning mechanisms separate to the frame to tension the printing screen.

This application is a division of U.S. patent application Ser. No.10/513,452 filed Nov. 2, 2004 now U.S. Pat. No. 8,069,783, which is anational phase of International Application No. PCT/GB03/01858 filedApr. 30, 2003, which claims priority of United Kingdom PatentApplication Nos. 0210102.0 filed May 2, 2002 and 0225130.4 filed Oct.29, 2002, all of which applications are hereby incorporated herein byreference.

The present invention relates to printing screens, often alternativelyreferred to as stencils or masks, for use in the screen printing ofworkpieces, typically electronic substrates, such as circuit boards andcomponents, frames therefor, and printing screen units.

In one known printing screen unit, the printing screen comprises a thin,stiff metal sheet, typically of stainless steel or brass, which includesa plurality of printing apertures which define a pattern for theprinting of a printing medium, typically solder paste. The printingscreen is mounted to a rectangular frame by a flexible, perforate sheet,typically a woven mesh of polypropylene or stainless steel strands,which acts to tension the printing screen.

Whilst the above-described printing screen unit has been well used, theunit does have a number of drawbacks, notably in being expensive tofabricate, and not allowing for the printing screen to be detached fromthe frame without permanently damaging the perforate sheet.

In another, more recent printing screen unit, the printing screen is ademountable printing screen which comprises a thin, stiff metal sheet,typically of stainless steel, which includes a plurality of printingapertures which define a pattern for the printing of a printing medium,typically solder paste, and a plurality of fixing apertures at oppositeedges thereof. The printing screen is mounted to a frame by mechanicaltensioning mechanisms which engage the fixing apertures in the printingscreen and act to tension the printing screen, with the tensioning forcetypically being provided by hydraulic, pneumatic or spring means.

WO-A-93/25061 discloses one such printing screen unit in which theprinting screen includes a plurality of elongate slots, as the fixingapertures, along each of opposite edges thereof, and the tensioningmechanisms each include a set of teeth which engage the slots atrespective ones of the opposite edges of the printing screen to tensionthe printing screen, with the tensioning mechanisms being configured todraw respective ones of the opposite edges of the printing screen overrespective supporting edges of the frame.

GB-A-2364961 discloses another such printing screen unit in which theprinting screen includes a plurality of first, fixing apertures alongeach of opposite edges thereof which are engaged by respectivetensioning mechanisms, and a plurality of second, flexing aperturesalong each of the opposite edges thereof which extend inwardly of therespective fixing apertures and provide for regions of increasedflexibility, with the tensioning mechanisms being configured to drawrespective ones of the opposite edges of the printing screen overrespective supporting edges of the frame.

These printing screen units, in not requiring the permanent attachmentof a printing screen to a frame, advantageously allow for printingscreens to be stored separately and mounted on frames as and whenrequired, thereby being cheaper and facilitating use.

These printing screen units do, however, still suffer from a number ofsignificant drawbacks. In particular, the edges of the printing screensare required to be more flexible than the main body thereof, and, assuch, are more prone to damage during handling and mounting. This isparticularly the case as the printing screens are stored as unsupportedsheets. Also, as the printing screens are very thin, typically having athickness of from about 0.1 to 0.25 mm, the edges thereof are razorsharp, which can lead to personal injury to a user if not handled withutmost care. Further, the printing screens are restricted to beingfabricated from materials which have the necessary strength to allow fordirect tensioning by tensioning mechanisms. Yet further, the printingscreens require specific manufacturing techniques, such as etching,electroforming and punching, and thereby limit the scope for massfabrication.

It is therefore an aim of the present invention to provide a printingscreen unit which at least partially overcomes the above-mentionedproblems associated with existing printing screen units, and printingscreens and frames for such printing screen units.

In one aspect, it is an aim of the present invention to provide ademountable printing screen which provides improved resistance to damageduring handling and mounting.

In another aspect, it is an aim of the present invention to provide ademountable printing screen which is safer to handle.

In a further aspect, it is an aim of the present invention to provide aprinting screen unit which allows for use of printing screens from awide range of materials, including plastics.

In a yet further aspect, it is an aim of the present invention toprovide a printing screen unit which can be manufactured cheaply andsimply.

In one aspect the present invention provides a printing screen,comprising a sheet, at least sections of at least one pair of oppositeedges of which are folded such as to define attachment elements.

Preferably, the attachment elements are each folded inwardly such as toenclose an acute angle with a major surface of the sheet.

In one embodiment at least ones of the attachment elements each comprisea plurality of projections spaced along a length thereof.

Preferably, the projections comprise rectangular projections.

Preferably, the projections of each attachment element define acastellated structure.

In another embodiment at least ones of the attachment elements eachcomprise a single, continuous attachment element, and the sheet includesa plurality of flexing apertures in spaced relation along each of theopposite edges thereof in zones overlapping fold lines of the respectivefolds.

In one embodiment the sheet has two pairs of opposite edges, and atleast sections of each of the pairs of opposite edges of the sheet arefolded such as to define attachment elements.

In one embodiment at least ones of the edges of the sheet have anattachment element extending along substantially the length thereof.

In another embodiment at least ones of the edges of the sheet have anattachment element extending along part of the length thereof.

Preferably, the part comprises a mid part.

In a further embodiment at least ones of the edges of the sheet haveattachment elements extending along parts of the length thereof.

Preferably, the parts comprise end parts.

In one embodiment the sheet includes a pattern of printing apertureswhich define a pattern of deposits to be printed thereby.

In another embodiment the sheet is a blank and includes no printingapertures therein.

In one embodiment the sheet comprises a metal sheet.

In another embodiment the sheet comprises a plastics sheet.

In another aspect the present invention provides a printing screen,comprising a sheet, at least sections of at least one pair of oppositeedges of which comprise a plurality of projections spaced along a lengththereof.

Preferably, the projections comprise rectangular projections.

Preferably, the projections define a castellated structure.

In one embodiment the sheet includes a pattern of printing apertureswhich define a pattern of deposits to be printed thereby.

In another embodiment the sheet is a blank and includes no printingapertures therein.

In one embodiment the sheet comprises a metal sheet.

In another embodiment the sheet comprises a plastics sheet.

In a further aspect the present invention provides a printing screenunit, comprising: a printing screen, comprising a sheet at leastsections of at least one pair of opposite edges of which includeattachment elements; and at least one pair of interface membersattachable to the attachment elements at the at least one pair ofopposite edges of the sheet.

Preferably, the at least sections of the at least one pair of oppositeedges of the sheet are folded such as to define attachment elements.

More preferably, the attachment elements are each folded inwardly suchas to enclose an acute angle with a major surface of the sheet.

In one embodiment at least ones of the attachment elements each comprisea plurality of projections spaced along a length thereof.

Preferably, the projections comprise rectangular projections.

Preferably, the projections of each attachment element define acastellated structure.

In another embodiment at least ones of the attachment elements eachcomprise a single, continuous attachment element, and the sheet includesa plurality of flexing apertures in spaced relation along each of theopposite edges thereof in zones overlapping fold lines of the respectivefolds.

In one embodiment at least ones of the edges of the sheet have anattachment element extending along substantially the length thereof.

In another embodiment at least ones of the edges of the sheet have anattachment element extending along part of the length thereof.

Preferably, the part comprises a mid part.

In a further embodiment at least ones of the edges of the sheet haveattachment elements extending along parts of the length thereof.

Preferably, the parts comprise end parts.

In one embodiment the printing screen unit comprises: a pair ofinterface members attachable to the attachment elements at one pair ofopposite edges of the sheet.

In another embodiment the sheet has two pairs of opposite edges; and theprinting screen unit comprises: a pair of interface members attachableto the attachment elements at one pair of opposite edges of the sheet;and further comprises: a pair of support members attachable to the otherpair of opposite edges of the sheet.

In a further embodiment the sheet has two pairs of opposite edges, atleast sections of which include attachment elements; and the printingscreen unit comprises: first and second pairs of interface membersattachable to the attachment elements at each of the pairs of oppositeedges of the sheet.

Preferably, the printing screen unit further comprises: corner piecescoupling adjacent ends of each of the interface members such as todefine a frame.

More preferably, the corner pieces each comprise one of first and secondrecesses or projections and the ends of the interface members eachinclude the other of recesses or projections, which projections andrecesses are engaged such as to provide for coupling of the interfacemembers.

Yet more preferably, the recesses and projections of the corner piecesand the interface members are configured such as to provide for relativemovement of the interface members on being tensioned by a tensioningmechanism.

More preferably, at least one of the corner pieces comprises at leastfirst and second coupling members which are configured such as to enableassembly thereof with the projections and recesses of the at least onecorner piece and interface members in engagement, thereby enablingassembly of the frame with the interface members thereof attached to therespective edges of the sheet.

In one embodiment the sheet includes a pattern of printing apertureswhich define a pattern of deposits to be printed thereby.

In another embodiment the sheet is a blank and includes no printingapertures therein.

In one embodiment the sheet comprises a metal sheet.

In another embodiment the sheet comprises a plastics sheet.

In one embodiment the interface members each include an attachmentsection which provides for attachment to the at least one attachmentelement at a respective edge of the sheet, and an engagement section forengagement to a tensioning mechanism.

Preferably, the attachment section of each interface member includes anattachment surface which in use is attached to the at least oneattachment element at a respective edge of the sheet.

More preferably, the attachment surface of each interface member isinclined inwardly from a free edge of the attachment surface relative tothe respective edge of the sheet.

More preferably, the attachment surface of each interface member isdefined by a continuous surface.

Preferably, the engagement section of each interface member includes anengagement surface for engagement to a tensioning mechanism.

More preferably, the engagement surface of each interface member isinclined outwardly from a free edge of the engagement surface relativeto the respective edge of the sheet.

More preferably, the engagement surface of each interface member isdefined by a continuous surface.

In yet another aspect the present invention provides a frame forsupporting a printing screen comprising a sheet having two pairs ofopposite edges, at least sections of which include attachment elements,the frame comprising: first and second pairs of interface membersattachable to the attachment elements at each of the pairs of oppositeedges of the sheet; and corner pieces coupling adjacent ends of each ofthe interface members such as to define the frame.

Preferably, the corner pieces each comprise one of first and secondrecesses or projections and the ends of the interface members eachinclude the other of recesses or projections, which projections andrecesses are engaged such as to provide for coupling of the interfacemembers.

More preferably, the recesses and projections of the corner pieces andthe interface members are configured such as to provide for relativemovement of the interface members on being tensioned by a tensioningmechanism.

Preferably, at least one of the corner pieces comprises at least firstand second coupling members which are configured such as to enableassembly thereof with the projections and recesses of the at least onecorner piece and interface members in engagement, thereby enablingassembly of the frame with the interface members thereof attached to therespective edges of the sheet.

In one embodiment the interface members each include an attachmentsection which provides for attachment to the at least one attachmentelement at a respective edge of the sheet, and an engagement section forengagement to a tensioning mechanism.

Preferably, the attachment section of each interface member includes anattachment surface which in use is attached to the at least oneattachment element at a respective edge of the sheet.

More preferably, the attachment surface of each interface member isinclined inwardly from a free edge of the attachment surface relative tothe respective edge of the sheet.

More preferably, the attachment surface of each interface member isdefined by a continuous surface.

Preferably, the engagement section of each interface member includes anengagement surface for engagement to a tensioning mechanism.

More preferably, the engagement surface of each interface member isinclined outwardly from a free edge of the engagement surface relativeto the respective edge of the sheet.

More preferably, the engagement surface of each interface member isdefined by a continuous surface.

In a yet further aspect the present invention provides a printing screenunit, comprising: a printing screen, comprising a sheet; and at leastone pair of interface members attached to at least one pair of oppositeedges of the sheet.

In one embodiment the interface members are bonded to the printingscreen.

Preferably, each interface member includes an engagement surface forengagement to a tensioning mechanism.

More preferably, the engagement surface of each interface member isinclined outwardly from a free edge of the engagement surface relativeto the respective edge of the sheet.

More preferably, the engagement surface of each interface member isdefined by a continuous surface.

In one embodiment the engagement surface is disposed outwardly of therespective edge of the sheet.

In another embodiment the engagement surface is disposed inwardly of therespective edge of the sheet.

In one embodiment the interface members each comprise a plurality ofinterface elements spaced along a length of the respective edge of thesheet.

In another embodiment the interface members each comprise first andsecond strips attached to opposite sides of the respective edge of thesheet.

In a further embodiment the interface members each comprise first andsecond strips attached to opposite sides of the respective edge of thesheet, and include a plurality of engagement apertures for engagement toa tensioning mechanism spaced along a length thereof.

Preferably, the sheet has two pairs of opposite edges, at least sectionsof which include attachment elements; and the printing screen unitcomprises: first and second pairs of interface members attached to theat least one attachment element at each of the pairs of opposite edgesof the sheet.

The present invention provides an improved printing screen unit which iscompatible with existing support frame systems, with the interfacemembers protecting edges of the printing screen from damage duringhandling and mounting, thereby increasing the life of the printingscreen. Further, sharp edges of the printing screen are enclosed, makingthe printing screen safer to handle. In addition, the interface membersconfer structural support, reducing the flex in the printing screenduring storage.

Significantly, the provision of interface members allows plastic sheetto be used as the primary material of printing screens for mounting ontensionable support frames.

Still further, the attachment of a printing screen to interface membersfurther simplifies the manufacture of the printing screen unit as awhole, in obviating the need for an adhesive bond.

Preferred embodiments of the present invention will now be describedhereinbelow by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1( a) illustrates a perspective view of a printing screen unit inaccordance with a first embodiment of the present invention;

FIG. 1( b) illustrates one corner region of the printing screen unit ofFIG. 1( a);

FIG. 1( c) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 1( a);

FIG. 2( a) illustrates one corner region of a printing screen unit inaccordance with a second embodiment of the present invention;

FIG. 2( b) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 2( a);

FIG. 3( a) illustrates one corner region of a printing screen unit inaccordance with a third embodiment of the present invention;

FIG. 3( b) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 3( a);

FIG. 4( a) illustrates one corner region of a printing screen unit inaccordance with a fourth embodiment of the present invention;

FIG. 4( b) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 4( a);

FIG. 5( a) illustrates a perspective view of a printing screen unit inaccordance with a fifth embodiment of the present invention;

FIG. 5( b) illustrates one corner region of the printing screen unit ofFIG. 5( a);

FIG. 5( c) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 5( a);

FIG. 6( a) illustrates a perspective view of a printing screen unit inaccordance with a sixth embodiment of the present invention;

FIG. 6( b) illustrates one corner region of the printing screen unit ofFIG. 6( a);

FIG. 6( c) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 6( a);

FIG. 7( a) illustrates a perspective view of a printing screen unit inaccordance with a seventh embodiment of the present invention;

FIG. 7( b) illustrates one corner region of the printing screen unit ofFIG. 7( a);

FIG. 7( c) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 7( a);

FIG. 8( a) illustrates a perspective view of a printing screen unit inaccordance with an eighth embodiment of the present invention;

FIG. 8( b) illustrates one corner region of the printing screen unit ofFIG. 8( a);

FIG. 8( c) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 8( a);

FIG. 9( a) illustrates a perspective view of a printing screen inaccordance with a ninth embodiment of the present invention;

FIG. 9( b) illustrates one corner region of the printing screen of FIG.9( a);

FIG. 9( c) illustrates a vertical sectional view through an edge regionof the printing screen of FIG. 9( a);

FIG. 10( a) illustrates a perspective view of a printing screen unit inaccordance with a tenth embodiment of the present invention;

FIG. 10( b) illustrates the upper side of one corner region of theprinting screen unit of FIG. 10( a);

FIG. 10( c) illustrates the under side of the one corner region of theprinting screen unit of FIG. 10( a);

FIG. 10( d) illustrates an exploded perspective view of the one cornerregion of the printing screen unit of FIG. 10( a);

FIG. 10( e) illustrates a vertical sectional view through an edge regionof the printing screen unit of FIG. 10( a);

FIG. 10( f) illustrates an exploded perspective view of one corner pieceof the printing screen unit of FIG. 10( a);

FIG. 11 illustrates one corner region of an alternative printing screenfor the printing screen unit of FIG. 10( a), with the attachmentelements being partially cut away for purposes of illustration; and

FIG. 12 illustrates an alternative corner piece for the printing screenunit of FIG. 10( a).

FIGS. 1( a) to (c) illustrate a printing screen unit 10 in accordancewith a first embodiment of the present invention.

The printing screen unit 10 comprises a printing screen 12, in thisembodiment of rectangular shape, a pair of interface members 16, 16which are attached to one pair of opposite edges of the printing screen12, and a pair of support members 18, 18 which are attached to the otherpair of opposite edges of the printing screen 12.

The printing screen 12 includes a pattern of apertures 20 which definethe pattern of deposits to be printed. The pattern of apertures 20 canbe formed, for example, by laser cutting or etching, as is well known inthe art. In preferred embodiments the printing screen 12 comprises asheet of a metal, such as stainless steel, or a plastic.

The interface members 16, 16 each comprise an elongate member whichextends along the length of the respective edge of the printing screen12.

The interface members 16, 16 each include an attachment section 22 whichextends inwardly of the respective free edge of the printing screen 12,as illustrated in FIG. 1( c), and is in this embodiment bonded to theprinting screen 12. The bonding of the interface members 16, 16 to theprinting screen 12 may be by any suitable adhesive or curing technique,as would be known to a person skilled in the art.

The interface members 16, 16 each further include an engagement section24 which, relative to the printing screen 12, is disposed outwardly ofthe attachment section 22 and provides for engagement to a tensioningmechanism.

The engagement section 24 includes an inwardly-facing engagement surface28, in this embodiment defined by a continuous channel, which extendsalong the length of the respective interface member 16. The engagementsurface 28 is outwardly inclined from the free edge thereof relative tothe printing screen 12, such as to define a hook arrangement whichprovides for captive engagement with a tensioning mechanism. As will beappreciated, the engagement surface 28, in being defined by a continuouschannel, allows for fabrication by a wide variety of manufacturingtechniques, such as machining, routing and extrusion, which are muchsimpler and cheaper techniques than the fabrication techniques requiredin fabricating the printing screens of the prior art printing screenunits.

The support members 18, 18 each comprise an elongate strip which extendsalong the length of the respective edge of the printing screen 12. Thesupport members 18, 18 provide both that no free, sharp edges of theprinting screen 12 are exposed, thereby preventing the risk of injuryduring handling, and also further support the printing screen 12,rendering the printing screen 12 less prone to flexing when beinghandled and stored. Furthermore, in providing the printing screen 12with additional structural support, the printing screen 12advantageously allows more readily for use of a plastic sheet. The useof plastic sheets has many advantages, in being cheaper, easier tofabricate, and lighter. As described hereinabove, metal sheets, such asstainless steel sheets, necessarily have to be used for existingprinting screens which are mounted directly to tensionable supportframes, as the design of those existing printing screens is not such asto withstand the tension applied thereto by tensionable support frames.

In one alternative embodiment the support members 18, 18 can be omittedfrom the printing screen unit 10.

In another alternative embodiment the printing screen unit 10 couldinclude a second pair of interface members 16, 16 attached to respectiveones of the other pair of opposite edges of the printing screen 12.

FIGS. 2( a) and (b) illustrate a printing screen unit 10 in accordancewith a second embodiment of the present invention.

The printing screen unit 10 of this embodiment is very similar to theprinting screen unit 10 of the above-described first embodiment, andthus, in order to avoid unnecessary duplication of description, only thedifferences will be described in detail, with like parts beingdesignated by like reference signs.

The printing screen unit 10 of this embodiment differs from that of thefirst-described embodiment only in the manner of attachment of theprinting screen 12 to the interface members 16, 16.

In this embodiment the interface members 16, 16 each include an elongatecover strip 30 which is bonded over the respective free edge of theprinting screen 12 such as to enclose the same. The bonding of theprinting screen 12 to the main bodies and the cover strips 30 of theinterface members 16, 16 can be by any suitable adhesive or curingtechnique, as would be known to a person skilled in the art.

FIGS. 3( a) and (b) illustrate a printing screen unit 10 in accordancewith a third embodiment of the present invention.

The printing screen unit 10 of this embodiment is quite similar to theprinting screen unit 10 of the above-described first embodiment, andthus, in order to avoid unnecessary duplication of description, only thedifferences will be described in detail, with like parts beingdesignated by like reference signs.

The printing screen unit 10 of this embodiment differs from that of thefirst-described embodiment only in the manner of attachment of theprinting screen 12 to the interface members 16, 16.

In this embodiment the edges of the printing screen 12 to which theinterface members 16, 16 are attached are folded to define attachmentelements 32, 32, and the attachment section 22 of each interface member16 includes a narrow, attachment slot 34 which extends along the lengththereof and in which a respective one of the attachment elements 32, 32is a frictional fit. In this embodiment the edges of the printing screen12 are folded such that the attachment elements 32, 32 defined therebyenclose an acute angle with the main body of the printing screen 12, andthe attachment slots 34 in the interface members 16, 16 are inclinedinwardly towards the respective inner edges thereof. In this embodimentthe interface members 16, 16 are slid onto the respective attachmentelements 32, 32 of the printing screen 12 and held thereto by thefrictional engagement therebetween, with there being no bonding thereof.In one embodiment the interface members 16, 16 could be bonded to theattachment elements 32, 32 of the printing screen 12. With thisconfiguration, the attachment elements 32, 32 become more firmly engagedin the attachment slots 34 in the respective interface members 16, 16 ontensioning the printing screen unit 10.

FIGS. 4( a) and (b) illustrate a printing screen unit 10 in accordancewith a fourth embodiment of the present invention.

The printing screen unit 10 of this embodiment is quite similar to theprinting screen unit 10 of the above-described first embodiment, andthus, in order to avoid unnecessary duplication of description, only thedifferences will be described in detail, with like parts beingdesignated by like reference signs.

The printing screen unit 10 of this embodiment differs from that of thefirst-described embodiment in the manner of attachment of the printingscreen 12 to the interface members 16, 16 and by the omission of thesupport members 18, 18.

In this embodiment the edges of the printing screen 12 to which theinterface members 16, 16 are attached are folded to define attachmentelements 32, 32, and the attachment section 22 of each interface member16 includes an attachment groove 36 which extends along the lengththereof, in this embodiment defined by a continuous channel, in which arespective one of the attachment elements 32, 32 is located. In thisembodiment the edges of the printing screen 12 are folded such that theattachment elements 32, 32 defined thereby enclose an acute angle withthe main body of the printing screen 12. In this embodiment theattachment groove 36 in the attachment section 22 of each interfacemember 16 defines an attachment surface 38 at the inner edge thereofrelative to the printing screen 12. The attachment surface 38 isinwardly inclined from the free edge thereof relative to the printingscreen 12, such as to define a hook arrangement which provides forattachment with a respective one of the attachment elements 32 of theprinting screen 12. In being inwardly inclined, the attachment surface38 of each attachment groove 36 provides for the captive attachment ofthe respective attachment element 32 of the printing screen 12 ontensioning the printing screen unit 10.

FIGS. 5( a) to (c) illustrate a printing screen unit 40 in accordancewith a fifth embodiment of the present invention.

The printing screen unit 40 comprises a printing screen 42, in thisembodiment of rectangular shape, and a pair of interface members 46, 46which are attached to one pair of opposite edges of the printing screen42.

The printing screen 42 includes a pattern of apertures 50 which definethe pattern of deposits to be printed. The pattern of apertures 50 canbe formed, for example, by laser cutting or etching, as is well known inthe art. In preferred embodiments the printing screen 42 comprises asheet of a metal, such as stainless steel, or a plastic.

The edges of the printing screen 42 to which the interface members 46,46 are attached are folded to define attachment elements 52, 52. In thisembodiment the edges of the printing screen 42 are folded such that theattachment elements 52, 52 defined thereby enclose an acute angle withthe main body of the printing screen 42.

The interface members 46, 46 each comprise an elongate member whichextends along the length of the respective edge of the printing screen42.

The interface members 46, 46 each include an attachment section 54 whichincludes an attachment slot 56 which extends along the length thereofand in which a respective one of the attachment elements 52, 52 of theprinting screen 42 is located.

The attachment slot 56 comprises a first, support slot part 56 a, inthis embodiment of linear section, which extends inwardly into theattachment section 54 from the inner edge of the respective interfacemember 46, and receives the region of the printing screen 42 which isinwardly adjacent the respective attachment element 52 of the printingscreen 42 and acts to support the respective interface member 46 in therequired orient relative to the printing screen 42.

The attachment slot 56 further comprises a second, attachment slot part56 b which extends inwardly and backwardly from the distal end of thesupport slot part 56 a such as to define a hook arrangement whichprovides for attachment with the respective attachment element 52 of theprinting screen 42. In being inwardly inclined, the attachment slot part56 b of the attachment slot 56 provides for the captive attachment ofthe respective attachment element 52 of the printing screen 42.

The interface members 46, 46 each further include an engagement section58 which provides for engagement to a tensioning mechanism.

The engagement section 58 includes an inwardly-facing engagement surface59, in this embodiment defined by a continuous rebate, which extendsalong the length of the respective interface member 46. The engagementsurface 59 is outwardly inclined from the free edge thereof relative tothe printing screen 42, such as to define a hook arrangement whichprovides for captive engagement with a tensioning mechanism. As will beappreciated, the engagement section 58, in being defined by a continuousrebate, allows for fabrication by a large variety of manufacturingtechniques, such as machining, routing and extrusion, which are muchsimpler and cheaper techniques than the fabrication techniques requiredin fabricating the printing screens of the prior art printing screenunits.

In an alternative embodiment the printing screen unit 40 could include asecond pair of interface members 46, 46 attached to respective ones ofthe other pair of opposite edges of the printing screen 42.

FIGS. 6( a) to (c) illustrate a printing screen unit 60 in accordancewith a sixth embodiment of the present invention.

The printing screen unit 60 comprises a printing screen 62, in thisembodiment of rectangular shape, and a pair of interface members 66, 66which are attached to one pair of opposite edges of the printing screen62 and extend inwardly thereof.

The printing screen 62 includes a pattern of apertures 70 which definethe pattern of deposits to be printed. The pattern of apertures 70 canbe formed, for example, by laser cutting or etching, as is well known inthe art. In preferred embodiments the printing screen 62 comprises asheet of a metal, such as stainless steel, or a plastic.

The interface members 66, 66 each comprise an elongate member whichextends along the length of the respective edge of the printing screen12.

The interface members 66, 66 each include an attachment section 72 whichis located at the respective free edge of the printing screen 62, asillustrated in FIG. 6( c), and is in this embodiment bonded to theprinting screen 62. The bonding of the interface members 66, 66 to theprinting screen 62 may be by any suitable adhesive or curing technique,as would be known to a person skilled in the art.

The interface members 66, 66 each further include an engagement section74 which provides for engagement to a tensioning mechanism.

The engagement section 74 includes an inwardly-facing engagement surface78, in this embodiment defined by a continuous surface, which extendsalong the length of the respective interface member 66. The engagementsurface 78 is outwardly inclined from the free edge thereof relative tothe printing screen 62, such as to define a hook arrangement whichprovides for captive engagement with a tensioning mechanism. As will beappreciated, the engagement surface 78, in being defined by a continuoussurface, allows for fabrication by a wide variety of manufacturingtechniques, such as machining, routing and extrusion, which are muchsimpler and cheaper techniques than the fabrication techniques requiredin fabricating the printing screens of the prior art printing screenunits.

In one alternative embodiment the printing screen unit 60 could includea second pair of interface members 66, 66 attached to respective ones ofthe other pair of opposite edges of the printing screen 62.

In another alternative embodiment the interface members 66, 66 can beconfigured such as to extend beyond the respective edges of the printingscreen 62 to which the interface members 66, 66 are attached. In oneembodiment the interface members 66, 66 can each include, as in thesecond-described embodiment, an elongate cover strip which is bondedover the respective free edge of the printing screen 62 such as toenclose the same.

FIGS. 7( a) and (b) illustrate a printing screen unit 60 in accordancewith a seventh embodiment of the present invention.

The printing screen unit 60 of this embodiment is quite similar to theprinting screen unit 60 of the above-described sixth embodiment, andthus, in order to avoid unnecessary duplication of description, only thedifferences will be described in detail, with like parts beingdesignated by like reference signs.

The printing screen unit 60 of this embodiment differs from that of thesixth-described embodiment only in the construction of the interfacemembers 66, 66.

In this embodiment the interface members 66, 66 each comprise aplurality of block elements 66′ which are spaced along the lengths ofthe respective edges of the printing screen 62 to which the interfacemembers 66, 66 are attached.

In one embodiment, where the printing screen 62 is formed of a plasticsmaterial, the block elements 66′ of the interface members 66, 66 couldbe machined or formed into a uniform thick plastic sheet.

FIGS. 8( a) to (c) illustrate a printing screen unit 80 in accordancewith an eighth embodiment of the present invention.

The printing screen unit 80 comprises a printing screen 82, in thisembodiment of rectangular shape, and first and second pairs of interfacemembers 86, 86, 87, 87 which are attached to the respective pairs ofopposite edges of the printing screen 82 and together define asupporting frame.

The printing screen 82 includes a pattern of apertures 90 which definethe pattern of deposits to be printed. The pattern of apertures 90 canbe formed, for example, by laser cutting or etching, as is well known inthe art. In preferred, embodiments the printing screen 82 comprises asheet of a metal, such as stainless steel, or a plastic.

The interface members 86, 86, 87, 87 each comprise a pair of elongatestrips 86 a, 86 b, 87 a, 87 b which extend along the length of therespective edge of the printing screen 82, with the strips 86 a, 86 b,87 a, 87 b being disposed on opposite sides of the printing screen 82and overlapping the respective edge of the printing screen 82 so as tosandwich the same.

The interface members 86, 86, 87, 87 each include an attachment section92 which extends inwardly of the respective free edge of the printingscreen 82, as illustrated in FIG. 8( c), and is in this embodimentbonded to the printing screen 82. The bonding of the interface members86, 86, 87, 87 to the printing screen 82 may be by any suitable adhesiveor curing technique, as would be known to a person skilled in the art.

The interface members 86, 86, 87, 87 each further include an engagementsection 94 which provides for engagement to a tensioning mechanism.

The engagement section 94 includes a plurality of engagement apertures96 which are spaced along the length of the respective interface member86, 86, 87, 87 and provide for engagement to a tensioning mechanism.

In this embodiment the printing screen unit 80 includes interfacemembers 86, 86, 87, 87 which extend along each of the edges of theprinting screen 82 and together define a frame which supports theprinting screen 82, but in an alternative embodiment the printing screenunit 80 can include only a single pair of interface members 86, 86 whichextend along one pair of opposite edges of the printing screen 82.

As mentioned hereinabove, in providing the printing screen 82 withstructural support through the provision of interface members 86, 86,87, 87, the printing screen 82 advantageously allows more readily foruse of a plastic sheet. The use of plastic sheets has many advantages,in being cheaper, easier to fabricate, and lighter. As again mentionedhereinabove, metal sheets, such as stainless steel sheets, have to datenecessarily been used for existing printing screens which are mounteddirectly to tensionable support frames, as the design of those existingprinting screens is not such as to withstand the tension applied theretoby tensionable support frames.

FIGS. 9( a) to (c) illustrate a printing screen 102 in accordance with aninth embodiment of the present invention.

The printing screen 102, in this embodiment of rectangular shape,includes a pattern of apertures 104 which define the pattern of depositsto be printed. The pattern of apertures 104 can be formed, for example,by laser cutting or etching, as is well known in the art. In a preferredembodiment the printing screen 102 comprises a sheet of a metal, such asstainless steel.

The edges of the printing screen 102 to which tensioning mechanisms areto be attached each comprise a plurality of spaced projections 106 alongthe length thereof which define a castellated structure, and are foldedto define attachment elements 108, 108. In this embodiment the edges ofthe printing screen 102 are each folded along a line intersecting theprojections 106, here at substantially a mid point of the length of theprojections 106, with each of projections 106 defining a hook element.In this embodiment the edges of the printing screen 102 are folded suchthat the attachment elements 108, 108 defined thereby enclose an acuteangle with the main body of the printing screen 102.

The projections 106 which define the castellated structures can, forexample, be formed using a press tool or laser. Alternatively, theprojections 106 could be formed by electro-discharge machining (EDM) orwater jet milling, which fabrication techniques would allow severalprinting screens 102 to be fabricated simultaneously.

The edges of the printing screen 102 can be folded by using, forexample, a roll or press tool.

In this embodiment, the configuration of the folded projections 106,which define the castellated structures, advantageously provides for theapplication of a uniform tension over the main body of the printingscreen 102 in taking up the shape of the tensioning mechanisms, andthereby prevents distortion of the main body of the printing screen 102,which typically has to be maintained flat.

FIGS. 10( a) to (f) illustrate a printing screen unit 110 in accordancewith a tenth embodiment of the present invention.

The printing screen unit 110 comprises a printing screen 112, in thisembodiment of rectangular shape, and first and second pairs of interfacemembers 116, 116, 117, 117 which are attached to the respective pairs ofopposite edges of the printing screen 112 and coupled by corner pieces118 a-d such as to define a supporting frame.

The printing screen 112 includes a pattern of apertures 120 which definethe pattern of deposits to be printed. The pattern of apertures 120 canbe formed, for example, by laser cutting or etching, as is well known inthe art. In preferred embodiments the printing screen 112 comprises asheet of a metal, such as stainless steel, or a plastic.

The edges of the printing screen 112 to which tensioning mechanisms areto be attached each comprise a plurality of spaced projections 122 alongthe length thereof which define a castellated structure, and are foldedto define attachment elements 124, 124. In this embodiment the edges ofthe printing screen 112 are each folded along a line intersecting theprojections 122, here at substantially a mid point of the length of theprojections 122, with each of projections 122 defining a hook element.In this embodiment the edges of the printing screen 112 are folded suchthat the attachment elements 124, 124 defined thereby enclose an acuteangle with the main body of the printing screen 112.

The projections 122 which define the castellated structures can, forexample, be formed using a press tool or laser. Alternatively, theprojections 122 could be formed by electro-discharge machining (EDM) orwater jet milling, which fabrication techniques would allow severalprinting screens 112 to be fabricated simultaneously.

The edges of the printing screen 112 can be folded by using, forexample, a roll or press tool.

In this embodiment, the configuration of the folded projections 122,which define the castellated structures, advantageously provides for theapplication of a uniform tension over the main body of the printingscreen 112 in taking up the shape of the tensioning mechanisms, andthereby prevents distortion of the main body of the printing screen 112,which typically has to be maintained flat.

The interface members 116, 116, 117, 117 each comprise an elongatemember which extends along the length of the respective edge of theprinting screen 112.

The interface members 116, 116, 117, 117 each include an attachmentsection 126 which comprises a first, lower body part 128 and a second,upper body part 130 and together define an attachment slot 132 whichextends along the length thereof and in which a respective one of theattachment elements 124 of the printing screen 112 is located.

The attachment slot 132 comprises a first, support slot part 134, inthis embodiment of linear section, which extends inwardly into theattachment section 126 from the inner edge of the respective interfacemember 116, 116, 117, 117, and receives the region of the printingscreen 112 which is inwardly adjacent the respective attachment element124 of the printing screen 112 and acts to support the respectiveinterface member 116, 116, 117, 117 in the required orient relative tothe printing screen 112.

The attachment slot 132 further comprises a second, attachment slot part136 which includes an attachment surface 138 which extends inwardly andbackwardly from the distal end of the support slot part 134 such as todefine a hook arrangement which provides for attachment with therespective attachment element 124 of the printing screen 112. In beinginwardly inclined, the attachment surface 138 provides for the captiveattachment of the respective attachment element 124 of the printingscreen 112.

The interface members 116, 116, 117, 117 each further include anengagement section 140 which, relative to the printing screen 112, isdisposed outwardly of the attachment section 126 and provides forengagement to a tensioning mechanism.

The engagement section 140 includes an inwardly-facing engagementsurface 142, in this embodiment defined by a continuous surface, whichextends along the length of the respective interface member 116, 116,117, 117. The engagement surface 142 is outwardly inclined from the freeedge thereof relative to the printing screen 112, such as to define ahook arrangement which provides for captive engagement with a tensioningmechanism. As will be appreciated, the engagement section 140, in beingdefined by a continuous surface, allows for fabrication by a widevariety of manufacturing techniques, such as machining, routing andextrusion, which are much simpler and cheaper techniques than thefabrication techniques required in fabricating the printing screens ofthe prior art printing screen units.

The interface members 116, 116, 117, 117 each further include recesses144, 146, in this embodiment of substantially rectangular section, atthe distal ends thereof for receiving the tongue elements 160, 164 ofrespective corner pieces 118 a-d, as will be described in more detailhereinbelow.

In this embodiment the corner pieces 118 a-d each comprise first andsecond coupling members 152, 154, which lock together to define therespective corner pieces 118 a-d.

In this embodiment the first coupling member 152 comprises a bodyelement 156 which includes a slot 158, here of the same rectangularsection as the recesses 144, 146 in the distal ends of the interfacemembers 116, 116, 117, 117, which extends in a first directiontherethrough, and a tongue element 160 for fitting in a respective oneof the recesses 144, 146 in the distal ends of the interface members116, 116, 117, 117 which extends from the body element 156 in a seconddirection orthogonal to the first direction of the slot 158 therein.

In this embodiment the second coupling member 154 comprises a bodyelement 162, and a tongue element 164 for fitting in an adjacent one ofthe recesses 144, 146 in the distal ends of the interface members 116,116, 117, 117 which extends from the body element 162. The body element162 of the second coupling member 154 is configured to be a tight,snap-fit in the slot 158 in the body element 156 of the first couplingmember 154 such that, when inserted therein, the tongue elements 160,164 of the coupling members 152, 154 are locked in a couplingconfiguration. In one embodiment the coupling members 152, 154 could bebonded together.

With this configuration, the coupling members 152, 154 allow for the insitu assembly of the last of the corner pieces 118 a-d to be fitted, inenabling the tongue elements 160, 164 of the respective pair of thecoupling members 152, 154 to be inserted into respective recesses 144,146 in the distal ends of the interface members 116, 116, 117, 117 inassembling the last corner piece 118 a-d. It will be appreciated thatthree of the corner pieces 118 a-d can be pre-assembled and need not beassembled in situ. Indeed, in one embodiment three of the corner pieces118 a-d can be fabricated as an integral unit, typically as a mouldingor casting.

In this embodiment the tongue elements 160, 164 of the coupling members152, 154 are configured such as to be a tight fit in the recesses 144,146 in the distal ends of the interface members 116, 116, 117, 117, butsuch as to allow for movement of the tongue elements 160, 164 in therespective recesses 144, 146 in the direction of a tension as applied tothe printing screen 112. With this configuration, the interface members116, 116, 117, 117 present a substantially rigid frame when handled, butallow for relative movement of respective pairs of the interface members116, 116, 117, 117 on being tensioned by tensioning mechanisms. Inallowing for relative movement of the interface members 116, 116, 117,117, design tolerances can be accommodated in order to provide foruniform tensioning of the printing screen 112.

In one alternative embodiment, as illustrated in FIG. 11, the printingscreen 112 can include single, continuous attachment elements 124 alongeach of the edges thereof, with the printing screen 112 including aplurality of flexing apertures 166 in spaced relation along each of thefold lines of the folds. This configuration, similarly to thecastellated structures, advantageously provides for the application of auniform tension over the main body of the printing screen 112 and yetallows for flexing of the printing screen 112 in taking up the shape ofthe tensioning mechanisms.

In another alternative embodiment, as illustrated in FIG. 12, the cornerpieces 118 a-d each comprise first and second coupling members 172, 174,which each comprise a tongue element 176 for fitting in a respective oneof the recesses 144, 146 in the distal ends of the interface members116, 116, 117, 117 and a keying structure 178 which provides for thelocking of the coupling members 172, 174 in a coupling configuration,first and second capping members 180, 182 which are disposed to oppositesides of the coupling members 172, 174, and fixing pins 184 for fixingthe capping members 180, 182 to the coupling members 172, 174 such as tofix the coupling members 172, 174 in the coupling configuration. In thisembodiment the capping members 180, 182 are also bonded to the couplingmembers 172, 174.

In this embodiment the coupling members 172, 174 are fabricated from asheet material and are of such a thickness as to allow for the in situassembly of the last of the corner pieces 118 a-d, in enabling thetongue elements 176 of a respective pair of the coupling members 172,174 to be inserted into respective recesses 144, 146 in the distal endsof the interface members 116, 116, 117, 117 in overlapping relation andsubsequent keying together of the keying structures 178 thereof in themanner of a jigsaw.

Finally, it will be understood that the present invention has beendescribed in its preferred embodiments and can be modified in manydifferent ways without departing from the scope of the invention asdefined by the appended claims.

For example, in one modification, the folded edges of the printingscreens 12, 42 of the third to fifth embodiments could utilize thecastellated edge structures of the ninth-described embodiment.

Also, in the described embodiments which utilize interface members 16,46, 66, 86, the interface members 16, 46, 66, 86 are described asextending along substantially the complete length of the respectiveedges of the printing screens 12, 42, 62, 82. In other embodiments theinterface members 16, 46, 66, 86 can be attached only to part or partsof the respective edges of the printing screens 12, 42, 62, 82, forexample, a mid part or end parts, with the interface members 16, 46, 66,86 merely extending along the other parts or part such as to protectboth the edges of the printing screens 12, 42, 62, 82 from damage and auser from the sharp edges of the printing screens 12, 42, 62, 82.

The invention claimed is:
 1. A printing screen unit, comprising: aprinting screen, comprising a sheet having two pairs of opposite edges,at least sections of which include attachment elements; and a frameincluding first and second pairs of interface members attachable to theattachment elements at the respective pairs of opposite edges of thesheet and corner pieces coupling adjacent ends of each of the interfacemembers, wherein the frame holds the printing screen captive in anuntensioned state when no tension is applied thereto by externaltensioning mechanisms separate to the frame, the frame having no elementor device mounted thereto nor carried thereby for maintaining theprinting screen in a tensioned state when no tension is applied theretoby external tensioning mechanisms separate to the frame, and the frameis substantially rigid when in the untensioned state, but allows forrelative movement of the respective pairs of interface members by atension applied by external tensioning mechanisms separate to the frameto tension the printing screen.
 2. The printing screen unit of claim 1,wherein the sheet is a flat sheet.
 3. The printing screen unit of claim1, wherein the at least sections of each pair of opposite edges of thesheet are folded such as to define attachment elements.
 4. The printingscreen unit of claim 3, wherein the attachment elements are each foldedinwardly such as to enclose an acute angle with a major surface of thesheet.
 5. The printing screen unit of claim 3, wherein at least ones ofthe attachment elements each comprise a plurality of projections spacedalong a length thereof.
 6. The printing screen unit of claim 5, whereinthe projections comprise rectangular projections.
 7. The printing screenunit of claim 5, wherein the projections of each attachment elementdefine a castellated structure.
 8. The printing screen unit of claim 3,wherein at least ones of the attachment elements each comprise a single,continuous attachment element, and the sheet includes a plurality offlexing apertures in spaced relation along each of the opposite edgesthereof in zones overlapping fold lines of the respective folds.
 9. Theprinting screen unit of claim 1, wherein at least ones of the edges ofthe sheet have an attachment element extending along substantially thelength thereof.
 10. The printing screen unit of claim 1, wherein atleast ones of the edges of the sheet have an attachment elementextending along part of the length thereof.
 11. The printing screen unitof claim 1, wherein the corner pieces each comprise one of first andsecond recesses or projections and the ends of the interface memberseach include the other of recesses or projections, which projections andrecesses are engaged such as to provide for coupling of the interfacemembers.
 12. The printing screen unit of claim 11, wherein the recessesand projections of the corner pieces and the interface members areconfigured such as to provide for relative movement of the interfacemembers on being tensioned by a tensioning mechanism.
 13. The printingscreen unit of claim 11, wherein at least one of the corner piecescomprises at least first and second coupling members which areconfigured such as to enable assembly thereof with the projections andrecesses of the at least one corner piece and interface members inengagement, thereby enabling assembly of the frame with the interfacemembers thereof attached to the respective edges of the sheet.
 14. Theprinting screen unit of claim 1, wherein the sheet includes a pattern ofprinting apertures which define a pattern of deposits to be printedthereby.
 15. The printing screen unit of claim 1, wherein the sheet is ablank and includes no printing apertures therein.
 16. The printingscreen unit of claim 1, wherein the interface members each include anattachment section which provides for attachment to the attachmentelement at a respective edge of the sheet, and an engagement section forengagement to a tensioning mechanism.
 17. The printing screen unit ofclaim 16, wherein the attachment section of each interface memberincludes an attachment surface which in use is attached to theattachment element at a respective edge of the sheet.
 18. The printingscreen unit of claim 17, wherein the attachment surface of eachinterface member is inclined inwardly from a free edge of the attachmentsurface relative to the respective edge of the sheet.
 19. The printingscreen unit of claim 16, wherein the engagement section of eachinterface member includes an engagement surface for engagement to atensioning mechanism.
 20. The printing screen unit of claim 19, whereinthe engagement surface of each interface member is inclined outwardlyfrom a free edge of the engagement surface relative to the respectiveedge of the sheet.
 21. The printing screen unit of claim 1, wherein thesheet is a stainless steel sheet.